In a rollicking day of spectacle, spite and scorn, Gov. Chris Christie of New Jersey declared his allegiance to Donald J. Trump and war on Marco Rubio, describing the senator on Friday as desperate and unfit for the presidency.

The endorsement interrupted a 48-hour assault from an emboldened Mr. Rubio, who is adopting many of the real estate mogul’s crude tactics and colorful insults as he urgently tries to arrest Mr. Trump’s march to the Republican nomination.

In the span of a few hours across Texas, Mr. Rubio suggested that Mr. Trump had urinated in his trousers and used illegal immigrants to tap out his unceasing Twitter messages. Mr. Trump countered by suggesting that Mr. Rubio’s excessive perspiration had no place in the White House and brandishing a water bottle to mock the senator’s chronic thirst.

Waving the bottle across a stage at a Texas rally, pouring half its contents onto the floor and then taking giant gulps from it, Mr. Trump ridiculed his younger rival with exaggerated facial gestures. “It’s Rubio!” he shouted to loud applause and cheers.

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Donald J. Trump mocked Marco Rubio’s chronic thirst by brandishing a water bottle Friday in Fort Worth, Tex. “It’s Rubio!” he shouted to cheers.Credit Cooper Neill for The New York Times

The escalation arrived four days before the biggest day of voting in the Republican primary campaign, and amid mounting alarm within the leadership of the party that its rank-and-file voters keep embracing a figure they view as hostile to their values and unworthy of their nomination.

Mr. Rubio seemed to relish his new role as a mischievous and combative saboteur, theatrically reading Mr. Trump’s misspelled online postings in front of an audience in Dallas and describing what he said was Mr. Trump’s descent into anxiety-ridden sweatiness backstage in between commercial breaks at their debate Thursday night.

“He was having a meltdown,” Mr. Rubio said, pacing a raised platform in what at times felt like a stand-up routine. “He was applying makeup around his mustache, because he had one of those sweat mustaches.”

The crowd roared.

Having tried and failed in so many ways to catch fire this campaign season, Mr. Rubio portrayed Mr. Trump as a con man who has tricked Republicans into believing he is an honest businessman and a genuine conservative.

“He is pulling the ultimate con job on the American people,” Mr. Rubio said. “It’s time to unmask him for what he is.”

But Mr. Rubio did not count on Mr. Christie, a totem of the Northeastern Republican establishment, bolting to Mr. Trump’s side on Friday and delivering the businessman’s biggest endorsement.

In doing so, Mr. Christie openly defied Republican attempts to isolate Mr. Trump as an unsavory party crasher and handed the front-runner an inexhaustible, media-savvy surrogate who on Friday pledged to travel the country campaigning for Mr. Trump and savaging his rivals.

Mr. Christie wasted little time in pursuing Mr. Rubio, a first-term Florida senator whom he holds in contempt for stealing his donors, kneecapping his campaign with brutal ads in New Hampshire and repeatedly surpassing him in the polls.

Brushing off Mr. Rubio’s pugnacious turn, Mr. Christie derided it as a fake, consultant-driven performance. “Part of his talking points now is to be entertaining and smile a lot now,” Mr. Christie said. “Listen, it’s one act after another.”

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Senator Marco Rubio at a rally on Friday in Dallas. Texas has the most delegates at stake on March 1, when 12 states hold nominating contests.Credit Cooper Neill for The New York Times

Behind the scenes, Mr. Trump had diligently courted Mr. Christie over the phone and in person over the past few weeks, despite the billionaire’s frequent dismissal of endorsements and the blandishments required to obtain them.

Mr. Trump and his wife, Melania, hosted Mr. Christie and his wife, Mary Pat, at Trump Tower in Midtown Manhattan for several hours on Thursday, according to a person briefed on the encounter. The talks apparently soothed whatever hurt remained from the pungent words the two men have used to describe each other: Mr. Christie has said that Mr. Trump lacks the “temperament” to be president, while Mr. Trump has accused Mr. Christie of blessing the scandalous lane closings on the George Washington Bridge that were engineered by the governor’s allies. (Mr. Christie has long denied any knowledge of the scheme, let alone a role in it.)

During a joint appearance in Fort Worth, Mr. Christie described his support as a gesture of loyalty to an old friend and an assessment of who stood the best chance of defeating the Democratic nominee in November.

But Mr. Trump seemed fixated, above all, on Mr. Christie’s talent for bludgeoning opponents such as Mr. Rubio, as he did during a Republican debate a few weeks ago by seizing on his penchant for robotic repetition. “I thought he was going to die,” Mr. Trump observed, as Mr. Christie stood to his side. “Good going, Chris,” he added.

Aghast party elders expressed dismay over the alliance, calling it a political marriage of expedience. “Good Lord almighty,” said Tom Ridge, the former governor of Pennsylvania and secretary of Homeland Security under President George W. Bush, who is backing Gov. John Kasich.

“Now it’s the walls and bridges team,” he quipped.

The timing of the Christie-Trump partnership immediately (and, it seemed, strategically) distracted from a growing liability for Mr. Trump: demands from Republican rivals and leaders that he release his tax returns, which Mr. Trump has rebuffed with claims that he is being audited.

As the scatological schoolyard taunts passed back and forth on Friday, slack-jawed voters struggled to contain their shock.

“I think Emily Post would be totally just turning in her grave right now,” said Tammy Ross, 52, after attending a rally for Mr. Rubio in Oklahoma City on Friday afternoon. “Anyone can say anything,” she tsk-tsked.

Cable news anchors fumbled for words sufficient to describe what they were hearing.

“Oh. My. God,” declared John Berman of CNN, after hearing Mr. Rubio’s vivisection of Mr. Trump in Dallas, which included this description of why the developer needed a full-length mirror backstage during the debate. “Maybe to make sure his pants weren’t wet. I don’t know,” Mr. Rubio cracked.

“It is on!” declared his startled co-host, Kate Bolduan.

As Mr. Trump and Mr. Rubio exchanged blistering broadsides, Senator Ted Cruz of Texas at times seemed to be an afterthought. Amid the caustic exchanges, Mr. Cruz discussed the finer points of federal water regulations during a taped interview in Nashville and engaged in an online war of words about immigration with Dennis Rodman, the retired basketball star.